1. Field
This invention is directed to recording media with enhanced magnetic properties for improved writability. In particular, the invention is directed to methods, systems and components that allow for improved writability while reducing defects so as to obtain uniform magnetic properties such as uniformly high anisotropy (Hk) and narrow switching field distribution (SFD).
2. Related Art
Magnetic media are widely used in various applications, particularly in the computer industry for data/information storage and retrieval applications, typically in disk form, and efforts are continually made with the aim of increasing the areal recording density, i.e., bit density or bits/unit area of the magnetic media. Conventional thin-film type magnetic media, wherein a fine-grained polycrystalline magnetic alloy layer serves as the active recording layer, are generally classified as “longitudinal” or “perpendicular” depending upon the orientation of the magnetic domains of the grains of magnetic material.
Typically, recording media are fabricated with polycrystalline CoCr or CoPt-oxide containing films. Co-rich areas in the polycrystalline film are ferromagnetic while Cr or oxide rich areas in the film are non-magnetic. Magnetic interaction between adjacent ferromagnetic domains is attenuated by nonmagnetic areas in between.
A conventional longitudinal recording, hard disk-type magnetic recording medium commonly employed in computer-related applications comprises a substantially rigid, non-magnetic substrate, typically of aluminum (Al) or an aluminum-based alloy, such as an aluminum-magnesium (Al—Mg) alloy, and a sequentially deposited or otherwise formed on a surface thereof a plating layer, such as of amorphous nickel-phosphorus (Ni—P); a bi-layer comprised of a seed layer of an amorphous or fine-grained material, e.g., a nickel-aluminum (Ni—Al) or chromium-titanium (Cr—Ti) alloy, and a polycrystalline underlayer typically of Cr or a Cr-based alloy; a magnetic recording layer, e.g., of a cobalt (Co)-based alloy with one or more of platinum (Pt), Cr, boron (B), etc.; a protective overcoat layer typically containing carbon (C), e.g., diamond-like carbon (“DLC”); and a lubricant topcoat layer, e.g., of a perfluoropolyether. Each of the substrate, plating layer, seed layer, interlayer, magnetic layer or overcoat may be deposited by suitable physical vapor deposition (“PVD”) techniques, such as sputtering, and the lubricant topcoat is typically deposited by dipping or spraying. A carbon overcoat is typically deposited in argon with nitrogen, hydrogen or ethylene. The protective overcoat protects the magnetic recording layer from corrosion and reduces frictional forces between the disc and a read/write head. The thin layer of lubricant may be applied to the surface of the protective overcoat to enhance the tribological performance of the head-disc interface by reducing friction and wear of the protective overcoat.
In operation of a longitudinal medium, the magnetic layer is preferably locally magnetized by a write transducer, or write “head”, to record and thereby store data/information therein. The write transducer or head can comprise a main (writing) pole as well as auxiliary poles and creates a highly concentrated magnetic field which alternates the media magnetization direction based on the bits of information to be stored. When the local magnetic field produced by the write transducer is greater than the coercivity of the material of the recording medium layer, the grains of the polycrystalline material at that location are magnetized. The grains retain their magnetization after the magnetic field applied thereto by the write transducer is removed. The direction of the magnetization matches the direction of the applied magnetic field. The magnetization of the recording medium layer can subsequently produce an electrical response in a read transducer, or read “head”, allowing the stored information to be read.
Efforts are continually being made with the aim of increasing the recording areal density and signal-to-medium noise ratio (“SMNR”) of the magnetic media. In this regard, so-called perpendicular recording media (recording media with a perpendicular anisotropy in the magnetic layer and magnetization forming in a direction perpendicular to the surface of the magnetic layers) have been found to be superior to the more conventional longitudinal media in achieving very high bit densities without experiencing the thermal stability limit associated with the latter. In perpendicular magnetic recording media, residual magnetization is formed in a direction (“easy axis”) perpendicular to the surface of the magnetic medium, typically a layer of a magnetic material on a suitable substrate.
Very high linear recording densities are obtainable by utilizing a “single-pole” magnetic transducer or “head” with perpendicular magnetic media. A typical perpendicular recording system utilizes a magnetic medium with a relatively thick (as compared with the magnetic recording layer) “soft” magnetic underlayer (SUL), a relatively thin “hard” perpendicular magnetic recording layer, and a single-pole head. Magnetic “softness” refers to a magnetic material having a relatively low coercivity below about 150 oersteds (Oe) or preferably below about 10 Oe, such as of a NiFe alloy (Permalloy) or a material that is easily magnetized and demagnetized. The magnetically “hard” recording layer has a relatively high coercivity of several kOe, typically about 2-10 kOe or preferably about 3-8 kOe, and comprises, for example, a cobalt-based alloy (e.g., a Co—Cr alloy such as CoCrPtB, or another material that neither magnetizes nor demagnetizes easily) having perpendicular anisotropy. The magnetically soft underlayer serves to guide magnetic flux emanating from the head through the hard, perpendicular magnetic recording layer. The system further preferably comprises a non-magnetic substrate, at least one non-magnetic interlayer, and an optional adhesion layer. The relatively thin interlayer comprised of one or more layers of non-magnetic materials, is preferably positioned below the at least one magnetically hard recording layer, and serves to prevent magnetic interaction between the soft underlayer and the magnetically hard recording layer and promote desired microstructural and magnetic properties of the hard recording layer. See US Publication No. 20070287031; U.S. Pat. No. 6,914,749; U.S. Pat. No. 7,201,977. The interlayer may comprise multiple layers forming an interlayer stack, with at least one of these layers preferably including an hcp (hexagonally close packed) material adjacent to the magnetically hard perpendicular recording layer.
Magnetic flux φ, emanates from the main writing pole of a magnetic transducer head, enters and passes through the at least one vertically oriented, magnetically hard recording layer in the region below the main pole, enters and travels within the SUL for a distance, and then exits therefrom and passes through the at least one perpendicular hard magnetic recording layer in the region below an auxiliary pole of the transducer head.
Granular perpendicular magnetic recording media is being developed for its capability of further extending the areal density of stored data, as compared to conventional perpendicular media, which is limited by the existence of strong lateral exchange coupling between magnetic grains. A granular (meaning that the in-plane grains are discontinuous in nature) perpendicular recording medium comprises a “hard” granular perpendicular magnetic layer having magnetic columnar grains separated by grain boundaries comprising voids, oxides, nitrides, non-magnetic materials, or combinations thereof. The grain boundaries, having a thickness of about 2 to about 20 angstroms, provide a substantial reduction in the magnetic interaction between the magnetic grains. In contrast to conventional perpendicular media, wherein the perpendicular magnetic layer is typically sputtered at low pressures and high temperatures in the presence of an inert gas, such as argon (Ar), deposition of the granular perpendicular magnetic layer is conducted at relatively high pressures and low temperatures and may utilize a reactive sputtering technique wherein oxygen (O) and/or nitrogen (N) containing molecules are introduced in a gas mixture of, for example, Ar and O2, Ar and N2, or Ar and O2 and N2, and H2O. Alternatively, oxides and/or nitrides may be introduced by utilizing a sputter target comprising oxides and/or nitrides, which is sputtered in the presence of an inert gas (e.g., Ar), or, optionally, may be sputtered in the presence of a sputtering gas comprising O and/or N with or without the presence of an inert gas. The introduced oxides and/or nitrides migrate into the grain boundaries and can provide for a granular perpendicular structure having a reduced lateral exchange coupling between grains. See US Publication No. 20060269797. The introduction of such grain boundaries can increase the areal density of recording/storing media.
The various layers within a medium described herein forms a stacked structure. The polycrystalline layers of the layer stack of the medium contain grain boundaries. Since a magnetically hard main recording layer and interlayer commonly comprise crystalline materials, and the hard magnetic layer is preferably grown coherently on the interlayer, grains of each polycrystalline layer are of substantially the same width (as measured in a horizontal direction) and in vertical registry (i.e., vertically “correlated” or aligned). Completing the layer stack is a protective overcoat layer, such as of a diamond-like carbon (DLC), formed over the hard magnetic layer, and a lubricant topcoat layer, such as of a perfluoropolyether material, formed over the protective overcoat layer. The perpendicular recording medium may also comprise a plating layer and/or a seed layer as described in the longitudinal recording medium configuration. The seed layer is preferably adjacent to the magnetically soft underlayer (SUL) and preferably comprises at least one of an amorphous material and a face-centered-cubic lattice structure (fcc) material. The term “amorphous” means that such a material exhibits no sharp peak in a theta-2theta X-ray diffraction pattern as compared to background noise. Amorphous layers may encompass nanocrystallites in amorphous phase or any other form of a material so long the material exhibits no peak in an X-ray diffraction pattern as compared to background noise. A seed layer seeds the nucleation of a particular crystallographic texture of the underlayer. Conventionally, a seed layer is the first deposited layer on the non-magnetic substrate. The role of this layer is to texture or align the crystallographic orientation of the subsequent Cr-containing underlayer. The seed layer, underlayer, and magnetic layer are conventionally sequentially sputter deposited on the substrate in an inert gas atmosphere, such as an atmosphere of argon.
Very fine-grained magnetic recording media may possess thermal instability. One solution is to provide stabilization via coupling of the ferromagnetic recording layer with another ferromagnetic layer or an anti-ferromagnetic layer. This can be achieved by providing a stabilized magnetic recording medium comprised of at least a pair of ferromagnetic layers which are anti-ferromagnetically-coupled (“AFC”). U.S. Pat. No. 6,777,112 is directed to interposing a non-magnetic spacer layer between AFC magnetic layers; however, inter-planar exchange coupling between grains of the recording medium layers has not been optimally controlled by prior art structures, and magnetic cluster size has not been reduced without sacrificing the thermal stability of the recording medium.
One configuration of a recording medium containing AFC-coupled recording layers comprises a continuous magnetic recording layer vertically stacked over a “hard” granular magnetic recording layer. The magnetic grains of the continuous layer are laterally more strongly exchange coupled while the magnetic grains of the hard granular layer are only weakly exchange coupled laterally. The continuous layer (often comprising a material having a relatively low coercivity or a material that is more easily magnetized and demagnetized) is ferromagnetically coupled to the hard granular layer (comprising a material having a relatively high coercivity or a material that neither magnetizes nor demagnetizes easily) in certain recording medium configurations. In such media, the entire continuous magnetic layer may couple with each grain in the granular hard magnetic layer underneath (forming a vertically exchange coupled composite—“ECC”). See U.S. Pat. No. 7,201,977.
The substrate is typically disk-shaped and may comprise glass, ceramic, glass-ceramic, NiP/aluminum, metal alloys, plastic/polymer material, ceramic, glass-polymer, composite materials non-magnetic materials, or a combination or a laminate thereof. See U.S. Pat. No. 7,060,376. A substrate material conventionally employed in producing magnetic recording rigid disks comprises an aluminum-magnesium (Al—Mg) alloy. Such Al—Mg alloys are typically electrolessly plated with a layer of NiP at a thickness of about 15 microns to increase the hardness of the substrates, thereby providing a suitable surface for polishing to provide the requisite surface roughness or texture. The optional adhesion layer, if present on the substrate surface, typically comprises a less than about 200 angstroms (Å) thick layer of a metal or a metal alloy material such as Ti, a Ti-based alloy, Ta, a Ta-based alloy, Cr, or a Cr-based alloy.
The relatively thick SUL is typically comprised of an about 30 to about 300 nm thick layer of a soft magnetic material such as Ni, Co, Fe, an Fe-containing alloy such as NiFe (Permalloy), FeN, FeSiAl, FeSiAlN, a Co-containing alloy such as CoZr, CoZrCr, CoZrNb, or a Co—Fe-containing alloy such as CoFeZrNb, CoFe, FeCoB, and FeCoC. The relatively thin interlayer stack typically comprises an about 50 to about 300 Å thick layer or layers of non-magnetic material(s). The interlayer stack includes at least one interlayer of an hcp material, such as Ru, TiCr, Ru/CoCr37Pt6, RuCr/CoCrPt, etc., adjacent the magnetically hard perpendicular recording layer. When present, a seed layer adjacent the SUL may typically include a less than about 100 Å thick layer of an fcc material, such as an alloy of Cu, Ag, Pt, or Au, or an amorphous or fine-grained material, such as Ta, TaW, CrTa, Ti, TiN, TiW, or TiCr. The at least one magnetically hard perpendicular recording layer is typically comprised of an about 10 to about 25 nm thick layer(s) of Co-based alloy(s) including one or more elements selected from the group consisting of Cr, Fe, Ta, Ni, Mo, Pt, W, Cr, Ru, Ti, Si, O, V, Nb, Ge, B, and Pd.
Of the conventional media types described above, longitudinal media are more developed than perpendicular media and have been utilized for several decades in the computer industry. During this interval, components and sub-systems, such as transducer heads, channels, and media, have been repeatedly optimized in order to operate efficiently within computer environments. However, it is a common current belief that longitudinal recording is reaching the end of its lifetime as an industry standard in computer applications owing to physical limits which effectively prevent further increases in areal recording density.
Perpendicular media, on the other hand, have replaced longitudinal media in many computer-related recording applications and continue the movement toward ever-increasing areal recording densities far beyond the capability of longitudinal media. However, perpendicular media and recording technology is less well developed than all facets of longitudinal media and recording technology. Specifically, each individual component of perpendicular magnetic recording technology, including transducer heads, media, and recording channels, is less completely developed and optimized than the corresponding component of longitudinal recording technology. As a consequence, the gains observed with perpendicular media and systems vis-à-vis the prior art, i.e., longitudinal media and systems, are difficult to assess.
High density perpendicular recording media require careful control and balance of several magnetic properties including: high enough anisotropy to enable thermal stability and compatibility with a high gradient head; low enough switching field to enable writability by the head; lateral exchange coupling low enough to maintain small correlation length between magnetic grains or clusters and high enough to maintain a narrow switching field distribution (SFD); and grain-to-grain uniformity of magnetic properties sufficient to maintain thermal stability and minimize SFD.
As recording density continues to increase, it is necessary to make smaller grain structures to maintain the number of magnetic particles in a bit at a similar value. Smaller grain structures are more sensitive to non-uniformities such as anisotropy variations within grains, and also require higher anisotropy to maintain thermal stability, thus adversely affecting writability. Therefore, there is a need in the art for a media with improved writability and fewer defects for narrower SFD and improved uniformity of properties.
Some current perpendicular recording media employ a recording layer including three or more Co-alloy magnetic layers to optimize magnetic parameters. For example U.S. Pat. No. 7,192,664 describes a first or bottom magnetic layer with a composition such as CoCr4-20Pt12-25(TiO2)4-12 that includes a fairly high oxide volume percentage for low Hex and a fairly high Pt concentration for high anisotropy. Other layers (for example, upper magnetic layers) may have lower oxide and Pt concentrations, and may contain other elements, e.g., B so as to tune exchange coupling, reduce SFD, and improve writability. See US Publication No. 20070064345. However, elements such as Pt and B can also introduce defects such as stacking faults that can degrade uniformity of properties such as anisotropy and limit the reduction of SFD, the narrowing of magnetic transitions, and the improvement of media signal-to-noise ratio (SNR). US Publication No. 20060024530.
In view of the foregoing, there exists a clear need for improved recording media, particularly perpendicular recording media, and system technology which are designed to functionally provide a range of benefits and performance enhancements as compared to conventional recording media and systems.